The present invention relates generally to welded tubular steel products, and more specifically to the production of electrical resistance welded alloy casing characterized by a high ultimate tensile strength and a relatively low yield strength comparable to that of plain carbon-manganese steels currently used.
A typical carbon-manganese steel consists essentially of about 0.33% carbon, 1.32% manganese, 0.30% silicon and the balance iron. These conventionally used steels require high normalizing temperatures of from 1700.degree. to 1750.degree. F. in order to approach minimum tensile strength requirements of about 95 ksi. Even when normalized at high temperatures, it has been difficult consistently to meet minimum tensile strength requirements in casing having wall thicknesses of about 3/8 inch and greater.
Attempts have been made to improve the tensile strength of carbon-manganese steels by alloying them with a number of elements such as molybdenum, vanadium, chromium, nickel, columbium, titanium and zirconium in varying amounts. In many instances, the higher tensile strengths of the alloyed steels were accompanied by increased yield strengths and inferior welding properties. It has also been found that the desired minimum strength requirement of 95 ksi could not be consistently attained in some of the alloyed steels when normalized at temperatures below about 1700.degree. F., this being particularly true in casing having wall thicknesses of about 3/8 inch or greater.